This invention is generally related to printheads for thermal inkjet printers and more particularly related to a thermal inkjet printhead having a predetermined site for the nucleation of thermally induced ink bubbles.
Thermal inkjet printing has become one of the standard techniques of transferring computer generated images or text onto tangible media such as paper or transparency film. Generally, a number of small orifices are arranged in such a fashion in a substrate that the expulsion of one or more droplets of ink from a determined number of orifices relative to a particular position of the medium results in the production of a portion (a pixel) of a desired character or image. Controlled repositioning of the substrate or the medium and another expulsion of ink droplets continues the production of more pixels of the desired character or image.
Expulsion of the ink droplet in a conventional thermal inkjet printer is a result of rapid thermal heating of the ink to a temperature which exceeds the boiling point of the ink solvent and creates a gas phase bubble of ink. Each orifice is coupled to a small unique chamber filled with ink and having an individually addressable heating element in thermal contact with the ink. As the bubble nucleates and expands, it displaces a volume of ink which is forced out of the orifice and deposited on the medium. The bubble then collapses and the displaced volume of ink is replenished from a larger ink reservoir.
It is desirable that the bubble be controlled in several aspects of its brief existence, including its rate of expansion, its ultimate volume, and its shape. The rate of expansion is primarily a function of the rate of heat energy input, the thermal properties of the ink, and the ambient temperature and pressure. The bubble volume is primarily related to the period of time the heat energy is input to the ink and the size of the firing chamber and heating device. The shape of the bubble is related to the physical configuration of the heating element and the shape of the ink chamber.
At the commencement of the heat energy output from the heating element, bubble nucleation generally commences at locations of dissimilarities in the ink liquid or at defect sites on the surface of the heating element or other interface surfaces (heterogeneous nucleation). It is well known that heterogeneous nucleation of a bubble is favored to occur energetically at interfaces. Although it is possible to promote homogeneous nucleation, it is not possible to do so in the absence of heterogeneous nucleation occurring at the interface between the ink and the contact surface where heat transfer occurs. Additional discussion regarding ink bubble formation for thermal inkjet printheads may be found in "Thermodynamics and Hydrodynamics of Thermal Inkjets" by Allen et al., Hewlett-Packard Journal, Vol. 36, No. 5, May 1985, pp. 21-27. If the location of these nucleation sites is not optimized, bubble formation will occur randomly or at various uncontrolled sites within the ink firing chamber. Therefore, although one may wish to drive the process to homogeneous nucleation on the heating surface of the structure, it is necessary to understand the interplay and negative aspects of heterogeneous nucleation which occurs due to its reduced energy requirement at the high energy interface. Earlier attempts at controlling bubble generation have concentrated upon spacing bubble generation away from cavitation-sensitive structures by construction of other low temperature structures or by overlaying discrete heat occluding devices on the passivation surface protecting the resistive layer. Each of these attempts, however, lack an integral surface layer which provides a favorable and controlled location for heterogeneous nucleation while maintaining a ruggedness of structure to withstand mechanical, chemical, and thermal stress associated with thermal inkjet printing. It can be appreciated, then, that, an apparatus which could control the bubble heterogeneous nucleation site would advantageously form a consistently located and well defined and reproducible bubble and produce a higher quality printed character or image.